Lower olefins, in particular olefins having from 2 to 4 carbon atoms, are suitable starting materials in a large number of chemical processes, including, for example, alkylation, oligomerization, and polymerization processes. The preparation of lower olefins from a hydrocarbon feed by cracking of that feed is a well-known process and is commercially applied at a large number of petrochemical manufacturing facilities. Typically, a distillate fraction of a crude oil, commonly a naphtha fraction of the crude oil, is used as the hydrocarbon feed in a naphtha cracker process to produce ethylene.
For commercial reasons, there is a demand for a naphtha cracking process having a high selectivity for lower olefins, in particular ethylene. There is also a demand to manufacture ethylene from hydrocarbon assets other than petroleum naphthas, especially ones that cost less and are more abundant. Examples of such hydrocarbon assets include natural gas, coal, and heavy oils found in abundant supply in locations that are remote from the ethylene markets. Currently there are two approaches to converting remote hydrocarbon assets into ethylene where the ethylene is manufactured in developed locations.
The first approach is to convert a hydrocarbon asset obtained at a remote site into a highly paraffinic feed by a Fischer-Tropsch process. This approach involves converting the hydrocarbon asset into synthesis gas by partial oxidation and converting the synthesis gas into a mixture of hydrocarbons by a Fischer-Tropsch process. A hydrocarbon fraction from the Fischer-Tropsch process may be used as a feed to a naphtha cracking process to produce ethylene. By way of example, European Patent Application No. 161705 discloses that a fraction of the product of a Fischer-Tropsch synthesis process may be used as a hydrocarbon feed in a naphtha cracking process. EP 161705 discloses using a C19− fraction from the Fischer-Tropsch process, wherein the C19− fraction substantially consists of linear paraffins, as a feed for a naphtha cracking process. EP 161705 further discloses that by using this feed, the selectivity toward lower olefins is increased compared with a naphtha fraction of a crude oil.
To increase the selectivity of the naphtha cracking process, a highly paraffinic Fischer-Tropsch naphtha that has been processed using hydrogen, including hydrotreating, hydrocracking, and hydroisomerization is typically used. To produce the ethylene, the highly paraffinic Fischer-Tropsch naphtha is typically shipped from the site where synthesized to a developed site and converted into ethylene in a naphtha cracker.
By way of example, “Performance of the SASOL SPD Naphtha as Steam Cracking Feedstock”, by Luis P. Dancuart, et al., ACS 2002 National Meeting, Boston Mass, Aug. 18–22, 2002, ACS Preprints Jul. 2002, and U.S. Pat. No. 5,371,308 describe examples of this approach. U.S. Pat. No. 5,371,308 teaches a process for preparing lower olefins from a hydrocarbon feed comprising a hydroprocessed synthetic oil fraction, wherein the hydrocarbon feed comprising the hydroprocessed synthetic oil fraction is cracked. The hydroprocessed synthetic oil fraction is derived from a synthesis process, such as a Fischer-Tropsch synthesis process and is subsequently treated in a process in the presence of hydrogen.
The second approach for converting a remote hydrocarbon asset into ethylene involves the production of methanol. This approach involves converting the hydrocarbon asset obtained at a remote site into synthesis gas by partial oxidation and converting the synthesis gas in a methanol synthesis plant into methanol. The methanol is typically shipped to a developed site and converted into ethylene by a Methanol-to-Olefins process. The methanol to olefins process uses a molecular sieve to dehydrate and convert the methanol to a mixture of ethylene, propylene and other olefins.
There are advantages to using the process involving Fischer-Tropsch naphtha to produce ethylene in comparison to the methanol process. These advantages include that the process involving Fischer-Tropsch naphtha can use existing conventional naphtha crackers. Also, the highly paraffinic naphtha produced in this process consists of a mixture of normal and iso-paraffins with few cyclic compounds (aromatics and naphthenes). This highly paraffinic naphtha provides higher yields of ethylene and lower coking rates than typical petroleum naphthas.
However, there are certain disadvantages of the process involving use of Fischer-Tropsch naphtha. The disadvantages include the high cost of converting methane into highly paraffinic naphtha. One element of this high cost is the hydrogen that is typically needed to hydrotreat the Fischer-Tropsch products to provide the highly paraffinic naphtha. In addition, the ethylene cracking step involves a high temperature endothermic reaction to dehydrogenate and crack the naphtha into smaller fragments. This high temperature endothermic reaction require the use of a significant amount of costly fuel.
The approach involving methanol synthesis may require fewer steps, but in general the economics of methanol production from natural gas are poor. In addition, when methanol is shipped, it must be remembered that approximately 50 wt % of the methanol is converted into water during the Methanol-to-Olefins step. Thus, approximately twice the amount of methanol must be shipped in comparison to a paraffinic naphtha. Furthermore, since methanol is toxic, it is typically shipped in small specialty tankers at higher costs than those needed for paraffinic naphthas. Finally, this approach requires the construction of new facilities for the Methanol-to-Olefins step.
There is a demand for economical and efficient processes to convert inexpensive hydrocarbon assets (such as methane or coal from remote sites) to ethylene in developed locations. It is desired that these processes have certain advantages. It is desired that the initial conversion of the hydrocarbon asset to the feed for the naphtha cracker be economical. It is desirable that the feed give high yields of ethylene thus requiring a smaller amount of feed initially. It is desirable that the naphtha cracking step have low operating costs. It is desirable that the overall process be compatible with existing facilities, including, for example, ships, tanks, pumps, naphtha crackers, etc.